Which is more predictable and rigged: US elections, or those in "non-democracies"?
Might that have something to do with turnout?

Predictability/Riggedness:

Over the decade 1995-2005,
The Political Oddsmaker by Ron Faucheux
(feature in Campaigns & Elections magazine)
has made over 2,700 picks in U.S. Senate, gubernatorial, U.S. House,
major mayoral and initiative elections, with an overall record of correctly predicting
winners over 98.2 percent of the time.
And I'm not talking about predictions based on looking at poll results 1 day
before the election, and only in selected easy-to-pick races.
No. I'm writing this in October 2005, and Faucheux
has already posted his searchable database of predictions for the 2006
elections, over 1 year ahead of time!

Think only Ron Faucheux is smart enough to do that?
The re-election rate for US congress seats is 98%.
That's far higher than any comparable democracy.
(E.g. compare Germany.)
Most important US races: Everybody knows who is going to win. It is all a foregone conclusion.

Compare with Communist China:

In China
the Communist Party nominates a candidate [for some local posts],
voters vote for him (runs unopposed) and if most voters approve, he is elected.
If most voters do not, the Party nominates somebody else and holds another
"election."
(Actually, there often now are multicandidate elections, the 1-candidate system
is now no longer predominant, see
this
and
this pdf; but for some
signs of backsliding see
this BBC story
10 Oct 2005 by Tim Luard,
and for an overview and perspective see the
Wikipedia
article. Some such system is employed in ≈1 million villages.)

The interesting thing about this crazy
system is that it actually, on average, where used, appears
to be more democratic than the United States, because in the USA there is over 98%
predictability over 1 year ahead of time, and generally no
"none of the above" (NOTA) option – which gives US voters about 2% (or
more correct is 4%) choice. In China
even if they get only 1 candidate, with NOTA they have some amount of genuine choice –
which probably usually amounts to more than 4%, although I have no statistics on how often
NOTA wins.
[Also the elections in China are a lot cheaper to run...]

Turnout:

Only about 50% of US voting-age adults voted
in Presidential election years since the 1980s.
In non-Presidential election years,
it's about 33%. In party primaries, it's about 25%.

Contrast:
In national elections since 1990,
67% of the British, 73% of Germans, 59% of Canadians, 60% of French, and 89%
of Italian
voting-age population cast ballots.
In the 2004 Spanish election (which empowered the Socialist Workers Party)
there was 77% electoral turnout.

In most European countries, more than two major parties compete.
(The more parties running, the greater the likelihood
that voters will find one that meets their preferences enough to draw them to the polls.
Also, the less chance everything is totally predictable.)

Europeans have a stronger labor movement. (I bet you've never even heard
of the US's labor party.)

By an 1845 law,
the USA has election day on a tuesday –
right in the middle of the work week.
In most other countries it's a weekend or holiday.